Patent application title: SURGE ARRESTER WITH A CAGE DESIGN, AND A PRODUCTION METHOD FOR IT

Abstract:

The invention relates to a surge arrester having a cage design, and to a
method for its production. According to the invention, during
extrusion-coating or encapsulation of a module comprising two end
fittings (3) and a plurality of varistor blocks (1) and at least one
reinforcing element (9) in order to form the outer housing (5), first
through-holes (11) for the reinforcing element (9) thereof are sealed
with silicone. For this purpose, second through-holes (15) are provided
in the end fittings (3), through which the silicone runs during casting
or spraying, and enters the through-holes (11) from the outside, in order
in this way to seal them against water and moisture.

Claims:

1. Surge arrester having:at least one varistor block (1);two end fittings
(3) which are arranged on opposite sides of the varistor block (1);at
least one reinforcing element (9), which holds the varistor block (1) and
the end fittings (3) together and runs through a first hole (11) through
at least one of the end fittings (3);an outer housing (5) composed of
silicone which is formed by casting or spraying around the varistor block
(1), the reinforcing element (9) and parts of the end fittings
(3);characterized in thatsecond through-holes (15) are formed in at least
one end fitting (3) and open in a groove (17) on that side of the end
fitting (3) which faces away from the varistor block (1), with the groove
(17) extending from a second through-hole (15) to a first through-hole
(11), and with the second through-hole (15), the groove (17) and the
first through-hole (11) being sealed with silicone on that side of the
end fitting (3) which faces away from the varistor block (1).

2. Surge arrester according to claim 1, characterized in thatthe varistor
block or blocks (1) is or are formed from a metal oxide, preferably zinc
oxide.

3. Surge arrester according to claim 1 or 2, characterized in thatthe end
fittings (3) are formed from metal, preferably aluminium.

4. Surge arrester according to one of the preceding claims, characterized
in that the housing (5) of the surge arrester is equipped with screens
(7).

5. Surge arrester according to one of the preceding claims, characterized
in thatthe reinforcing element (9) is a glass-fibre-reinforced plastic
rod or cable, which holds the end fitting (3) and the varistor block (1)
together with a tensile load.

6. Surge arrester according to claim 5, characterized in that an anchoring
element in the form of a wedge or a wedge sleeve holds the
glass-fibre-reinforced plastic rod (9) in the through-hole (11).

7. Method for production of a surge arrester according to claim 1, having
the following steps:mounting of at least one reinforcing element (9) in a
first through-hole (11) in a first end fitting (3);arrangement of a stack
of varistor blocks (1) on the end fitting (3) and alongside the
reinforcing element (9);fitting of a second end fitting on the stack of
varistor blocks and the reinforcing elements (9) in such a manner that
the varistor blocks (1) are located between the two end fittings
(3);mounting of the reinforcing element (9) in a first through-hole (11)
in the second end fitting (3);extrusion-coating or encapsulation of parts
of the two end fittings (3), of the varistor blocks (1) and of the
reinforcing element (9) in order to form an outer housing (5) composed of
plastic, with the plastic flowing through the second through-holes (15)
and the groove (17) into the end fittings (3), and with the first
through-holes (11) being sealed on that side of the end fittings (3)
which faces away from the varistor blocks (1).

Description:

[0001]The invention relates to a surge arrester having a cage design, as
is known by way of example from JP 62-149511 (application number), and to
a method for production of a surge arrester such as this.

[0002]Surge arresters are connected between live cables and earth in
electrical power supply systems in order, in the event of an overvoltage
on the cable, to dissipate this overvoltage to earth, and thus to protect
other components in the electrical power supply system. A surge arrester
such as this contains a stack of varistor blocks, which is held between
two connecting elements or end fittings. This arrangement is accommodated
in a housing.

[0003]In order to ensure that the varistor blocks make good contact with
one another even when mechanical loads are applied, it is necessary to
hold the stack together under pressure. In the case of surge arresters
having a cage design, this is done by means of reinforcing elements, in
general rods or cables, preferably glass-fibre-reinforced plastic rods
(GFC rods), which are held under tension at the two end fittings.

[0004]One problem with surge arresters such as these is to securely attach
the reinforcing elements to the end fittings so that the necessary
strength is maintained even when mechanical loads are applied, such as
those which occur when surge arresters are installed in the open air.

[0005]This problem is solved in the cited Japanese patent application by
the provision of grooves in the stacking direction of the varistor blocks
in the end fittings, into which the reinforcing elements are inserted,
with the end of the reinforcing elements being equipped with a thread
onto which a nut is screwed, whose diameter is larger than the groove in
the end fitting, thus essentially holding the reinforcing element by
means of an interlock.

[0006]According to a further known technique, as is disclosed in European
Patent EP 93 915 343.3, the glass-fibre reinforced plastic rods can also
be held on the end fittings by pins or screws, at right angles to the
longitudinal direction of the rods.

[0007]One problem that arises with the two known techniques is that the
projecting corners of the nuts and screws lead to a concentration of the
electrical field, and to the risk of partial discharges.

[0008]German Patent Application DE 199 40 939 indicates a further possible
way to hold the reinforcing elements in the end fittings. For this
purpose, a sleeve which tapers conically in the direction of the stack
centre of the varistor blocks and has moving side walls is inserted into
a conical hole in the end fitting and, acting as a clamping sleeve in a
similar manner to a wedge, holds the associated reinforcing element under
tension firmly by means of a friction-fitting or force-fitting
connection.

[0009]This technique makes it possible to avoid projecting edges of screws
or nuts, but one remaining problem is that moisture and/or water can
enter the interior of the surge arrester through the through-holes in the
end fittings, through which the glass-fibre-reinforced rods run and in
which the clamping sleeves are inserted, causing permanent damage to the
surge arrester.

[0010]With this technique, it was therefore necessary either to completely
integrate the end fittings in the design of an outer housing, for example
by also extrusion coating the end fittings, or it was necessary to take
additional measures in order to protect the end fittings against the
ingress of moisture on their upper face. These known techniques either
have the disadvantage that a large amount of relatively expensive
material is required for the outer housing, generally low-viscosity
silicone, in order to extrusion coat the entire end fitting, or an
additional process step is required in order to fit the additional seal.

[0011]The object of the invention is to provide a surge arrester having a
cage design, which is not subject to the problem of partial discharges
resulting from projecting edges of screws or nuts, and in which problems
resulting from the ingress of moisture through through-holes in the end
fittings do not occur either.

[0012]According to the invention, this problem is solved by a surge
arrester according to Claim 1 and by a method for production of a surge
arrester such as this according to Claim 7. The dependent claims relate
to further advantageous aspects of the invention.

[0013]Preferred embodiments of the surge arrester will be described in the
following text with reference to the drawings, in which:

[0015]FIG. 2 shows a view of the end fitting of the surge arrester
according to the invention;

[0016]FIG. 3 shows a section view through an end fitting as shown in FIG.
2.

[0017]The surge arrester which is shown in FIG. 1 and has a cage design
contains at least one varistor block 1. Known ceramic discs with a
voltage-dependent resistance (variable resistor) may be used as varistor
blocks 1. At low voltages, they operate as virtually perfect isolators,
while they have good conductivity at high voltage. Commercially available
varistor blocks are produced on the basis of zinc oxide (ZnO). However,
the invention is not restricted to zinc-oxide surge arresters such as
these, and other metal oxides as well as silicon carbide, for example,
may be used for the varistor block. Furthermore, in addition to varistor
blocks 1, it is also possible to use further blocks, for example metal
blocks or spark-gap blocks in the stack, in order in this way to match
the length of the surge arrester to the requirements of the respective
purpose.

[0018]Commercially-available varistor blocks 1 are in the form of circular
cylinders with a diameter of, for example, 5 cm and a height of about 4
cm. Aluminium electrodes, which are not shown in detail, are fitted to
both sides of the varistor blocks 1, in order to ensure a better contact.
It is also normal to place thin aluminium discs, which are likewise not
shown, or spring elements between the varistor blocks 1 in order to
further improve the contact.

[0019]In addition, a cup spring or the like can be provided in the stack
of varistor blocks, in order to ensure that the contact is maintained in
the event of temperature fluctuations.

[0020]A stack which is formed by stacking varistor blocks 1 such as these
and possibly metal blocks on top of one another is held between two end
fittings 3 in the surge arrester as shown in FIG. 1. The end fittings 3
are normally formed from aluminium or stainless steel and are designed in
such a manner that they can easily be included in existing electrical
installations or electrical power supply systems, for example by means of
a central screw 4 which projects out of the surge arrester and makes good
electrical contact with the varistor blocks 1.

[0021]For protection against the environment, these surge arresters are
surrounded by an outer housing 5 composed of silicone. The housing is
formed by spraying or casting.

[0022]Screens 7 are formed on the outside of the housing 5, in order to
increase the creepage distance for the current.

[0023]Surge arresters are subject to considerable bending moments when
they are used in an open-air environment. It is thus necessary to ensure
that, even when subjected to relatively large mechanical loads, the
contact between the varistor blocks 1 and with the end fittings is
maintained, and that edge fracture of the varistor blocks caused by
internal tilting of two adjacent varistor blocks is avoided. In order to
achieve this, glass-fibre-reinforced plastic rods or cables 9 are
normally clamped in between the two end fittings 3, as reinforcing
elements. These hold the varistor blocks 1 together between the two end
fittings 3, with a tensile load.

[0024]In the following text, the anchoring elements are referred to as
rods 9, although this should not be seen as any restriction to the
invention.

[0025]FIG. 2 shows a plan view of an end fitting for a surge arrester
according to the invention. The end fitting 3 is essentially in the form
of a circular-cylindrical block, whose diameter is greater than that of
the varistor blocks. Through-holes 11 which run in the stacking direction
are formed along the circumference of the end fittings in the radial area
of the end fitting, which projects beyond the varistor blocks. A further
through-hole 25 for the central screw 4 is formed, preferably with an
internal thread, in the centre of the end fitting.

[0026]In the case of the surge arrester according to the invention, the
glass-fibre-reinforced plastic rods 9 are held in holes 11 through the
end fittings 3. The glass-fibre-reinforced plastic rods are held firmly
in these through-holes by suitable means, such as wedges, wedge sleeves,
adhesive bonding, crimp sleeves or the like.

[0027]In addition, the end fitting has at least one second through-hole
15. This through-hole 15, in which no reinforcing element is held, is
used as a flow connection between the two sides of the end fitting while
the outer housing is being formed by spraying or casting.

[0028]Furthermore, in the case of the illustrated end fitting, that side
of the end fitting which faces away from the stack of varistor blocks is
provided with a circumferential groove or channel 17, in which the first
and the second through-holes 11 and 15 open. The groove is bounded by a
projecting outer rim 19 and an inner rim 21.

[0029]In order to prevent moisture from entering the interior of the surge
arrester from the outside through this through-hole, it is necessary to
take measures in order to seal the through-holes 11 against water.

[0030]In the case of the surge arrester according to the invention, the
cage is first of all formed from two end fittings 3, the varistor blocks
1 and the glass-fibre-reinforced rods 9, firmly connected to the end
fittings 3. This cage is then placed in a mould, forming a tight seal at
the respective shoulders 23 of the end fittings, as shown in FIG. 3. The
mould can be designed such that the screws 7 are also formed at the same
time as the outer housing 5, by spraying or casting with silicone.

[0031]Low-viscosity silicone as is preferred for this application is
relatively expensive. Thus, as can be seen in FIG. 1, the housing is
drawn in in the area of the varistor blocks 1, that is to say it has a
narrower cross section than in the area of the overlap with the end
fitting 3.

[0032]According to the invention, in addition to the first through-holes
11, in which the glass-fibre-reinforced rods 9 are accommodated, the
second through-holes 15 are also formed in the end fittings 3. That end
of these holes which faces the outside of the surge arrester is
incorporated in the groove 17, as has been explained. A plurality of
second through-holes are preferably provided. It is also possible to
provide a plurality of partial segment grooves instead of the continuous
circumferential groove 17, which each extend over only a segment of the
circumference, and with each first through-hole 11 together with at least
one second through-hole 15 opening in one of the partial segment grooves
17.

[0033]During the production of the surge arrester, a plate 17 is held by
means of a central screw 4 in the central hole with an internal thread 23
of the end fitting 3. The plate 27 shown in FIG. 1 ends flat with the rim
19 at the rim of the end fitting 3, forming a seal, with a cavity
remaining in the area of the groove 17. If required, individual vent
holes can be provided in the plate.

[0034]During the casting of the outer housing, one or more inlet openings
for the low-viscosity silicone are normally provided in the area of the
screens 7, and corresponding ventilation holes are provided, likewise in
the area of the screens 7. The silicone penetrates into the mould, forms
the outer housing 5 with the screens 7, in the process running through
the through-holes 15 and the grooves 17, and further into the
through-holes 11 with the glass-fibre-reinforced rods. This results in
the through-holes 11 being hermetically sealed from the outside with the
glass-fibre-reinforced plastic rods 9, and being protected against
moisture, in one process, with the formation of the outer housing.

[0035]As can be seen in FIG. 2, a second through-hole 15 can preferably be
provided for two first through-holes 11. This is not absolutely necessary
and it is possible to provide a second through-hole 15 for each first
through-hole 11, or a single second through-hole 15 can be provided for
all of the first through-holes 11.

[0036]The internal diameter of the second through-holes 15 can be chosen
in such a manner that the low-viscosity silicone can flow through them
during the casting process.

[0037]FIG. 3 shows a section through an end fitting along the line A-A in
FIG. 2. As can be seen, the second through-holes 15 have a considerably
smaller cross section than the first through-holes 11.

[0038]Although the invention has been described with reference to one
preferred embodiment, the invention is not restricted to this embodiment,
and those skilled in the art will identify various variations and
modifications within the scope of protection of the attached claims.